Controllable filler prefloculation using a dual polymer system

Abstract

A method of preparing a stable dispersion of flocculated filler particles for use in papermaking processes comprises sequential addition of high and low molecular weight flocculating agents to an aqueous dispersion of filler particles followed by shearing of the resultant filler flocs to the desired particle size resulting in shear resistant filler flocs with a defined and controllable size distribution.

Description

TECHNICAL FIELD[0001]This invention relates to the preflocculation of fillers used in papermaking, particularly, the production of shear resistant filler flocs with a defined and controllable size distribution at high filler solids is disclosed.BACKGROUND OF THE INVENTION[0002]Increasing the filler content in printing and writing papers is of great interest for improving product quality as well as reducing raw material and energy costs. However, the substitution of cellulose fibers with fillers like calcium carbonate and clay reduces the strength of the finished sheet. Another problem when the filler content is increased is an increased difficulty of maintaining an even distribution of fillers across the three-dimensional sheet structure. An approach to reduce these negative effects of increasing filler content is to preflocculate fillers prior to their addition to the wet end approach system of the paper machine.[0003]The term preflocculation means the modification of filler partic...

AI Technical Summary

Benefits of technology

[0008]The preflocculation process of this invention introduces a viscous flocculant solution into an aqueous filler slurry having a high solids content without causing significant flocculation by controlling surface charge of the filler particles. This allows the viscous flocculant solution to be distributed evenly throughout the high solids slurry. The second component, which is much less viscous than the flocculant solution, is introduced to the system to form stable filler flocs. This second component is a polymer with lower molecular weight and opposite charge compared to the flocculant. Optionally, a microparticle can be added as a third component to provide additional flocculation and narrow the floe size distribution. The floe size distribution is controlled by applying extremely high shear for a sufficient amount of time to degrade the floe size to the desired value. After this time, the shear rate is lowered and the floe size is maintained. No significant reflocculation occurs.

Problems solved by technology

However, the substitution of cellulose fibers with fillers like calcium carbonate and clay reduces the strength of the finished sheet.

Another problem when the filler content is increased is an increased difficulty of maintaining an even distribution of fillers across the three-dimensional sheet structure.

However, filler flocs formed by a low molecular weight coagulant alone, including commonly used starch, tend to have a relatively small particle size that breaks down under the high shear forces of a paper machine.

Filler flocs formed by a single high molecular weight flocculant tend to have a broad particle size distribution that is difficult to control, and the particle size distribution gets worse at higher filler solids levels, primarily due to the poor mixing of viscous flocculant solution into the slurry.

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